In the oilfield it has become common practice to drill a well that intersects numerous formations or portions of formations. Sometimes the well may be primarily vertical and sometimes the well may have a significant horizontal section. Once the wellbore has been drilled it is usually necessary to case the well. In the past the casing was typically a number of joints of solid pipe joined together and then run into the wellbore. Once the casing had been located in the wellbore it was then cemented in place by forcing cement through the interior of the pipe, out of the toe of the pipe, and back up around the annular area formed between the casing and the wellbore itself.
With the casing cemented in the well the interior of the pipe casing was effectively sealed from allowing any fluids to flow from the formations to the interior will. The typical practice to access the formations from the interior of the casing has been an operation referred to as plug and perf. In a plug and perf operation a bridge plug with the setting tool top of it and the perforating gun on top of the setting tool are run into the well. Once the bridge plug was located below the lower end of the desired formation zone the bridge plug was set by the setting tool thereby sealing the casing at the bridge plug and preventing any fluid from passing below the bridge plug. The setting tool is then released from the bridge plug in the setting tool and perf gun are raised some distance above the bridge plug. Once the perf gun is located adjacent to the formation to which access is desired to perf gun is fired. The perf gun is a set of shaped charges that when fired are able to pierce the casing and penetrate some distance past the casing into the formation thereby allowing fluid in the formation to flow to the interior of the casing and vice versa. Once the formation is accessed, the perf gun and setting tool are removed from the casing. Fluid is then pumped down the wellbore at high pressure, out through the perforations in the casing and into the formation, which in turn fractures the formation. Once the fracturing operation is complete the pumps at the surface are turned off. A new bridge plug setting tool and perf gun are assembled at the surface and then run into the casing. Once the second bridge plug is located below the second-highest formation, from the toe of the well, the bridge plug is set and the process is repeated until all of the various formations have been fractured. Once all of the formations are fractured, access to the lower formations through the bridge plug is necessary, therefore the usual practice is to run a drill back into the casing and drill out all of the intervening bridge plugs thereby allowing full bore access to all of the formations.
In order to avoid the costs associated with drilling out multiple bridge plugs, a slightly newer practice is to include a number of sliding sleeves in the casing before the casing is run into the well bore and cemented in place. Typically each sliding sleeve has a seat in the sliding sleeve. The seats are arranged so that the smallest diameter sliding sleeve seat is closest to the toe and the largest diameter sliding sleeve seat is closest to the surface. Each sliding sleeve is placed in the casing so that when the casing is run into the wellbore the appropriate sliding sleeve will be adjacent to the formation from which access is desired. When the operator then desires to fracture a particular formation a ball is pumped through the casing. The diameter of the ball is chosen so that it will pass through each of the larger diameter seats in the sliding sleeves closer to the surface but once it gets to the lowest sliding sleeves the ball will seat and allow no further fluid flow to pass the particular sleeve in which it is seated. Fluid pressure on the surface is then increased causing force to be exerted against the ball and its seat thereby opening the attached sliding sleeve. Once the sliding sleeve is open, the formation adjacent the sliding sleeve may then be fractured. After fracturing the formation a slightly larger diameter ball that corresponds to the seat in the next higher sleeve is pumped through the casing where the ball lands in the sleeve and the process is repeated until all of the sliding sleeves have been opened and formations fractured. After the fracturing operations are completed the balls may be allowed to flow out of the well or dissolve to allow access to the formations.
Unfortunately because the diameter of the casing has been restricted by the increasingly smaller diameters of the sleeve towards the toe of the well fracture pressure into the lower formations and production out of the lower formations is inhibited. Another issue that operators run into when they use progressively larger balls from the toe towards the heel is due to the diametric limitations of the number of balls that will fit, and hence a limited ability to be able to treat and access as many zones as possible from a single wellbore. In order to maximize the number of sliding sleeve that may be used in a well the variations from a smaller ball size to next larger ball size is kept as low as possible. Typically a ⅛ inch variation between ball sizes is seen. The limitation on size variation is due to the constraints posed by the material of the sliding sleeves ball seat, the ball itself, and the force applied to the ball and then transferred to the ball seat. For instance a ball seat may be cast-iron whereas the ball may be aluminum, plastic, composite, dissolvable, or other appropriate material. After the ball reaches the sleeve and lands on the seat pressure is applied against the ball and through the ball to the seat in order to overcome any biasing device and shift the sleeve open. However it must be kept in mind will that all of the force applied against the ball is transferred to the seat only through the balls ⅛ inch periphery that is in contact with the seat. Therefore only a limited amount of force may be applied to the ball before either the ball deforms or the periphery of the ball shears thereby allowing the ball to pass through the seat thus causing the failure of the particular sleeve.